When comparing Gmail vs Org mode for Emacs, the Slant community recommends Org mode for Emacs for most people. In the question“What are the best personal Knowledge Base apps?”Org mode for Emacs is ranked 1st while Gmail is ranked 8th. The most important reason people chose Org mode for Emacs is:

Emacs runs on all platforms, Windows, Linux, OSX, even Android and IOS

Pros

Pro

Best support for Gmail features

Since both the Gmail Android app and Gmail itself is made by Google, the app supports all features Gmail offers (like labeling/starring) and gets new features implemented early - usually as soon as they are announced.

Pro

Incredibly fast, advanced & predictive search

Gmail brings much of the functionality and power of Google's search engine to the app's search. You can use operators such as label:, is:, OR, has: among others, define date ranges, limit search to specific groups and so on. And, you can do it all amazingly fast. The search will also try to predict what you're trying to search for before you've even finished your query and offer up those results.

Pro

Comes bundled with Android for free

Not only is the application free, it comes bundled with the Android OS by default so you don't even have to bother downloading it.

Pro

Google Drive integration

The app allows inserting files from Drive straight to email or uploading attachments to Drive before sending them out. It will also notify if the person you're sending the file can open the file our should you change the sharing settings for that file beforehand. Finally, you can choose which one of your Google accounts you want visible in the app.

Pro

Tabs to automatically organize your e-mail

Each of the categories can be disabled and you can move your mail around from category to category or set your own rules which sender should be put in which category.

Pro

Google Calendar integration

Pro

Safely automatically displays images

The app does not require a confirmation to show images as Google now hosts the images on their own servers making it safe. You can still disable the automatic displaying of images in the settings if you wish.

Pro

Threaded conversations

Pro

Cross platform

Emacs runs on all platforms, Windows, Linux, OSX, even Android and IOS

Pro

Exports data to PDF, LaTeX, HMTL and other

You can even build presentation or blog from your data

Pro

Can be as simple or as complex as you want

Org mode started out as a simple outlining/note taking app. Then each outline heading can optionally be a TODO list item. And if you desire, you can add SCHEDULE and DEADLINE dates. (Schedule being the date you'd like to start the item.) You can customize the TODO states to add things like DELEGATED, WAITING, BLOCKED, etc.

Pro

Supports tables with formules (quite powerful!)

Pro

Inline image displaying

Pro

Uses Emacs

Pro

Libre/open source

Pro

Free but copyrighted

Cons

Con

Can't sort mail by Unread

Nearly every other mail app has this ability. If you have more than one Gmail/email account or get a lot of email, this is a significant omission. *You can sort other, non-Gmail​ accounts linked to the Gmail app, but not Gmail accounts.

Con

Very limited HTML composing

Composing of HTML formatted mails is very limited. No inline images and only some basic text formatting options

Con

Too heavy

It should be light weight.

Con

Limited notification buttons

Con

Uses Emacs (for those who prefer vi, menus, etc.)

Con

Complicated to get started

Con

Obtuse documentation

The documentation assumes a level of understanding of basic Emacs usage that not all users may have.